1. Field of the Invention
The present invention relates to a quantification method for remaining liver function, and a novel liver receptor imaging agent with liver targeting property.
2. Related Art
Asialoglycoprotein receptor (ASGPR) in the liver is known to specifically bind to glycopeptides or glycoproteins having Gal or GalNAc on an end. When liver lesion occurs, the number of ASGPR will change, but there is not a systematic quantitative evaluation tool that has been widely accepted. Therefore, it is desirable to develop glycopeptides or glycoproteins with a Gal or GalNAc end to serve as liver receptor imaging agent. The liver receptor imaging agent has the following utilities in the industry.
1. Liver transplantation often fails due to transient hypoxia, and whether the liver transplantation is successful or not can be immediately known through liver receptor imaging after the transplantation.
Liver receptor imaging is indicative of actual liver function. After binding with ASGPR, glycopeptides or glycoproteins with Gal and GalNAc end enter hepatocytes through receptor-mediated endocytosis. When liver lesion occurs, the liver receptor is reduced, and the imaging level will be reduced. Thus, the actual liver function can be evaluated by the imaging level theoretically.
2. Liver receptor imaging can be used to evaluate the anti-hepatitis and anti-fibrotic effects of Chinese herbal medicines.
3. The liver receptor imaging agent has highly specific liver targeting property, and can effectively carry medicines to be accumulated into liver at a concentrated dosage, so that not only the used dosage and treatment cost can be significantly reduced, but also the generation of side effects can be effectively reduced.
4. The liver receptor imaging agent has highly specific liver targeting property and is highly safe, and can be used as gene delivery vector for liver without any unnecessary allergic immune response.
This type of liver receptor imaging agents has the potential of quantifying the liver storage function. Meanwhile, there is an urgent need for an agent to determine the remaining liver function in clinic, to help clinicians to determine whether a liver transplantation is necessary or not for a patient. In Taiwan, there are 3,000,000 patients with hepatitis B, 500,000 patients with hepatitis C, and numerous patients with drug-induced hepatitis, all of whom are at high risk of liver failure, and need regular assessment of liver storage function.
Presently, the peptides or proteins to be polymerized with saccharide groups that have been disclosed in documents and patents include albumin, tyrosine-glutamyl-glutamic acid (YEE), tyrosine-aspartyl-aspartic acid (YDD), and tyrosine-glutamyl-glutamyl-glutamic acid (YEEE).
Tc-99m-Galactosyl-Serum-Albumin (Tc-99m GSA) is known as a liver receptor imaging agent, and has been used in clinic in Japan. But, GSA is a protein, and is a biological product having a very high molecular weight of about 67 kD. Therefore, it is difficult to know the position and the number of the saccharide group and DTPA connected to the protein exactly even with MS. In contrast, YEE, YDD, YEEE are peptides having a molecular weight of 1-2 kD. For them, the position and the number of the saccharide group and DTPA connected to the peptide can be predicted with MS, thus the quality control procedure is significantly simplified compared with GSA.
YEE and YDD are firstly set forth by Lee (1983), and YEEE is an improved invention by Chen (TW1240002, 2000). In 1983, Lee set forth that the binding force between galactosamine peptide with two chains in series and hepatocyte receptor is 1000 times of that of galactosamine peptide with a single chain, and the binding force between galactosamine peptide with three chains in series and hepatocyte receptor is 106 times of that of galactosamine peptide with a single chain. It is necessary to find a scaffold having at least three functional groups to polymerize three galactosamine chains together, for which a polymerized amino acid, i.e., peptide, is useful, for example, glutamyl-glutamic acid (EE, in which glutamic acid is represented as E), aspartyl-aspartic acid (DD, in which aspartic acid is represented as D), and lysine-lysine (KK, in which lysine is represented as K). Both EE and DD have three COOH functional groups being exposed and can thus be jointed with three galactosamine peptides having a certain length. Due to pKa in an acidic range, the product can be precipitated in an acid, thus facilitating separation and purification. As for KK, it has three amino groups and one COOH functional group, with the three amino groups being not easily linked to the saccharide chains, so it has not been used to develop liver receptor imaging agent till now.
In order to facilitate iodine isotope labeling, EE and DD are attached with Y (tyrosine), allowing in vivo imaging or cell receptor binding test. However, for the iodine labeling of YEE or YDD, it is necessary to add an oxidant, such as chloramine T, Iodobead, or Iodogen. In case of in vivo imaging, it is necessary to remove the oxidant by purification at the end of the reaction, because the oxidants are toxic to human body.